This invention relates generally to high voltage cables and insulators therefor employed in connection with cable termination and splicing or cable coupling and joining and more particularly to cable termination adaptors and their employment in cable couplers and the method of constructing the cable termination.
It is well known that the termination or jointure of high voltage shielded cables presents problems relating to the formation of corona and high concentration of electrical stresses. Cable termination by its nature necessitates the stripping of the cable's outer conductive layers as well as a portion of the conductor insulation for connection to an electrical contact or connector for ultimate communication with another contact or connector of another permanently joined or releasably coupled cable. Many methods of cable termination have been developed employing various types of termination adaptors having a stress relief cone, special conductor connectors, conductive inserts for termination adaptors, semiconductive shielding by means of coating or taping, etc., all aimed at redistribution of electrical stresses formed at the cable termination and thereby reducing the chances of corona formation and eventual breakdown of the insulation at the point of termination.
Many attempts have been made to eliminate as much as possible localized air pockets or voids anywhere along the cable terminus so as to reduce chances of the development of corona, that is, the ionization of air or gas entrapped within the elements making up the cable termination thereby initiating a discharge which will eventually break down the termination insulation causing cable failure. A side effect is the production of ozone which hastens the breakdown process of dielectric materials in the area of the cable terminus. By redistribution of the electrical stresses developed at the cable terminus, high stress areas can be effectively reduced to a point where the chance for development of corona at normal specified margin above operating levels is very minimal. This marginal level may be referred to as the corona extinction level, that is, the voltage level below which corona disappears, having established the actual voltage level at which corona is present. In actual practice, corona may be experienced during a voltage excursion resulting from a transient surge or fault condition.
Cable terminus in the past has always been with the objective of increasing the dielectric thickness of the cable adjacent the point where the cable shielding over the insulated conductor is actually terminated. This dielectric buildup usually takes the form of a cone and has the effect of redistributing the electrical stress concentration, that is, the concentration of the electrical field at the terminus to reduce the possibility of corona discharge and ultimate dielectric failure of the cable. Thus, a divergent potential gradient is introduced at the dielectric buildup area. The shield, in such cases, is effectively extended to end or terminate somewhere along the buildup area or cone so as to have a larger diametrical extent than that of the shielded cable.
The dielectric buildup was originally provided by means of a multiple layer of dielectric tape which was hand-wrapped on cable terminus. Also, self-bonding tape and different types of insulating gels and epoxies were used in an attempt to make the tape wrappings air-tight. Since these enlarged dielectric areas are generally done by hand in the field, it is virtually impossible to produce a completely air-tight, no-void terminus with multiple layers of wrapping tape, since the construction of the terminus depends largely upon workmanship and experience of the individual constructing the cable termination.
Premolded stress relief cones have since come into existence to eliminate problems brought about by multiple layer tape wrapping and further reduce the possibility of constructing a cable terminus with undesirable voids or air pockets which are most frequently present at surface irregularities along various dielectric layers making up the stress cone.
However, problems still remain with the expertise needed to produce a void-free cable termination since the skill developed through experience in using premolded adaptors is the main factor in determining the life of the cable connection until dielectric breakdown might possibly occur. Also, problems with such adaptors have been experienced with regard to tolerances of the internal bores of the semiconductive and insulative portions of such adaptors relative to the shielded insulated conductors upon which the adaptor is inserted. If there is too much tolerance, air pockets may be present either in the semiconductive or insulative portions of such adaptors. If there is too little tolerance, it becomes very difficult, if not impossible, to work the insulated conductor into the adaptor, whereas if it is simple to slide or insert the insulated conductor into the adaptor, it may be possible to slide the adaptor up over the semiconductive shielding covering the insulated conductor thereby creating an undesirable void in that region.
In high voltage cables where the insulation covering the conductor is comparatively thick, eccentricity of the insulation about the conductor is readily discernible and can give rise to problems in cable termination. The occurrence of eccentricity is particularly inherent in tandem extruded cables consisting of semiconductive layer, insulation and outer semiconductive jacket.
The extruded insulation and outer jacket of the cable are basically circular. However, the outer cylindrical surface of the cable is not concentric relative to the centrally located conductor. Thus, in employing a cable tool to remove out cable layers such as the outer semiconductive layer, more cable insulation will also be removed from one quadrant or side of the cable compared to the other in insuring no semiconductive material is left on the surface of the conductor insulation. This is because the cable conductor is concentrically located relative to the outer cable layers. By removing more of the insulation from one cable quadrant as compared to another because of this eccentricity, it will be evident to those knowledgeable in the high voltage cable termination art that a "step" or shoulder will be inevitably produced between the surface or the insulated conductor and the terminus of the outer semiconductive shield, which step or shoulder will also include in that one quadrant a contiguous "insulation step" which interferes with good cable termination. This is because the many termination adaptors of the prior art are employed concurrently with the formal shoulder giving rise readily to the formation of a "corona pocket" or region where corona can develop across a formed gap between the contiguous insulation step and the semiconductive position of the termination adaptor. Pencilling of this shoulder has been suggested which would require a termination adaptor of a different design.
In particular, if anything, the use of premolded adaptors may have created problems in producing rims, edges or shoulders with contiguous portions of the insulated conductor providing invitation to produce voids or air spaces in preparing or otherwise constructing a cable terminus. For example, U.S. Pat. No. 3,352,962 recognized the problem of entrapped air between the adaptor and the insulated conductor no matter how tight the relationship so that an annular cavity was provided to place any such entrapped air all in one location between the semiconductive adaptor and the insulated conductor. However, the problem of entrapped air within the cable terminus is not completely solved but rather collected to reside at a particular point within the cable terminus, with the result that electrical stresses developed across the trapped air may be somewhat reduced. In fact, the problem is still present in those situations where the cable semiconductive and insulative layers are not concentric about the central conductor, discussed above, so that the trapped air actually exists in practice between the semiconductive portions of the adaptor and the insulative covering of the conductor.
Historically, stress relief cones or cable terminating adaptors in general have provided a semiconductive portion in physical contact with the cable shield or semiconductive layer and the conductor insulation for a short length of the cable to provide for shield or semiconductive layer connection and termination as illustrated in U.S. Pat. Nos. 3,243,756; 3,352,962; and 3,378,627. However, the employment of adaptors in this manner necessitates proper and uniform termination of these outer conductive cable layers and precise positioning of the adaptor relative to the cable terminus of the semiconductive or shielding layer. Furthermore, ribs or shoulders are present at the ends of such adaptors, as applied to the cable, as previously stated, inviting the possible inclusion of voids.
In the past, to eliminate the problems encountered by employing highly elastomeric adaptors as illustrated in these patents, some have chosen to employ fairly rigidly constructed adaptors so as to insure that the end of the adaptor would not ride up over the terminus point of the cable semiconductive layer, preventing the possibility of a void area in the region which can be easily produced with highly elastomeric type adaptors. However, the employment of such adaptors made of relatively rigid material in some instances has not been satisfactory because they have become cracked usually in a direction perpendicular to the base of the adaptor for a substantial length of the adaptor, particularly of the semiconductive portion of the adaptor. These cracked conditions are usually created when inserting the adaptor over the shielded cable causing the material to spread and tear or be placed under such continuous tension conditions, as applied to the insulated conductor, to later crack or tear apart.
Further, these outer conductive layers of the cable obviously have greater diametrical extent than the insulated conductor so that if the insulation covering of the conductor is out of round or not concentric, these outer layers will be even more so out of round relative to the central conductor, which will be particularly exaggerated in the large diameter cables of the high voltage type where the insulation thickness is necessarily greater such as in a 69 kv or 138 kv cable. This is undesirable at a point of cable terminus because the redistribution of the electric stress at this point will also not be accomplished in a uniform manner. Covering these layers with a portion of the bore of the termination adaptor would merely compound the problem of providing for more uniform distribution of electrical stresses.
Thus, what those skilled in the art have been striving for is to prepare and construct a high voltage cable terminus with the minimum of materials, labor and expertise in the field producing a void-free termination including the provision of (1) an effective electrical stress relief cone with continuous extension of the outer conductive layers of the cable upon the cone; (2) increased dielectric strength along inner and outer creepage paths; (3) uniform gradual increase of outer concentric conductive layers from the point of terminus of these layers on the cable outwardly along the taper of the stress relief cone; (4) tighter tolerance of taping, coating or tubing conductive and insulative layers between the adaptor and the cable terminus of these layers as stripped to produce a satisfactory corona extinction level; and (5) above all, a cable terminus which does not take the skill and experience developed by one over the years in the field but can be easily constructed with a minimum of experience because of the reduction or otherwise the elimination of probable points within the cable terminus for eventual development of corona and subsequent dielectric failure.
In other words, it is the long felt need in the high voltage cable termination art, whether for permanent cable jointure or releasable cable coupling, to remove the present criticalness of preparing cable terminations in the field so that a corona free terminus might be provided for with a minimum of necessary skill.
Mention has only been made thus far relative to cable termination. However, equally important in the case where two high voltage cables are to be joined or coupled together is providing for a moisture or atmospheric tight, shockproof but releasably connectable cable coupler. Cable couplers of the past have not provided for weather tight connection nor have they been provided to be shockproof, having been previously provided with glass polyester insulators. These cable couplers experience very rough treatment in the field and must be constructed to take a certain amount of misuse so that an effective atmospheric or weather seal as well as secure conductor connection is maintained in spite of abuse in the field. Also, if the coupler employing the glass polyester type insulator assembly is dropped or otherwise misused, any internal breakage or damage to the assembly would have disastrous electrical consequences.
Thus, there is a large desire in the high voltage cable art to improve present couplers toward the elimination of insulator assembly breakage, corona induced ozone attack, increase the extent of creepage paths to ground, provide insulator assembly having higher dielectric strength materials and in general, provide a coupler assembly having higher corona extinction levels particularly for higher voltage class cable systems.